The impact of habitual dietary protein intake on the anabolic response in elderly men

During the adult life skeletal muscle mass remains fairly constant until the
fourth or fifth decade. Then, the slow process of sarcopenia (the age-related
loss of muscle mass) is believed to begin. The maintenance of skeletal muscle
mass is regulated by a balance between the opposing processes of muscle protein
synthesis and muscle protein breakdown. Food intake, dietary protein in
particular, stimulates muscle protein synthesis and allows net muscle protein
accretion throughout the day, which allows the normal maintenance of muscle
mass in healthy individuals. Many studies have described the postprandial
muscle protein synthetic response to protein intake and/or physical activity,
and these acute findings have led to recommendations for protein intake for
both athletes wishing to gain muscle mass as well as patients and elderly
individuals to help them maintaining muscle mass. However, translating the
acute findings from a single meal to long-term recommendations is perhaps
premature, since scientists know very little with regard to how previous
consumed meals affect the anabolic responsiveness to subsequent food intake. A
characteristic of the adaptation to habitual high or low protein intake is
thought to be associated with a change in the amplitude of diurnal cycle of
whole body proteins. If this speculation is accurate, it implies that the
muscle protein synthetic responses to feeding (differences between fasting and
feeding muscle protein synthesis rates) are adapting to differing habitual
protein intake, which may reduce (or enhance) the anabolic responsiveness to
protein intake.

To gain a more complete scientific understanding, it is necessary to examine
whether an adaptation does in fact occur after habitual high or low amounts of
protein intake with regard to the anabolic response to subsequent protein
intake. In the present investigation, we wish to investigate the impact of the
habitual consumption of either high or low protein diets for 14 days on the
anabolic responsiveness to a protein meal in healthy elderly.

single blind intervention study

Subjects will receive food bags for 14 days, containing either a low (0.7 g/kg
BW/d) or high (1.5 g/kg BW/d) protein diet. Following this (i.e. day 15), the
subjects will report to the laboratory in a fasted state and attend a single
experimental trial where they ingest 25 g of intrinsically labeled whey protein
in 350 mL water.

The burden and risks associated with participation are small. Insertion of the
catheters is comparable to a blood draw and could result in a small hematoma.
Muscle biopsies will be taken under local anesthesia by an experienced
physician, but may cause some minor discomfort for maximally up to 24 h after
completion. The discomfort is comparable to muscle soreness or the pain one has
after bumping into a table. We will take 5 blood samples (1x 10 mL and 4x 5 mL)
and 19 blood samples (8 mL) during the screening and experimental trial,
respectively. The total amount of blood we draw is less than half the amount of
a blood donation and will be completely restored in approximately 1 month. For
both the screening and the experimental trial, subjects have to be fasted, so
they are not allowed to eat and drink (except for water) from 22h00 the evening
before. Also, 3 prior to the experimental trial subjects should keep their diet
as constant as possible, do not perform any type of intense physical exercise,
and do not consume alcohol. Furthermore, we will ask the subjects to fill out a
dietary record for 3 random days prior to the experimental trial.
The stable isotope amino acids tracers applied in this experiment are not
radioactive and are completely safe. The production of the tracers for
intravenous administration will occur in a sterile environment according to GMP
guidelines.
There is no risk associated with the DEXA scan. The radiation dose emitted
during a DEXA scan is 0.001 mSv. This is a very low exposure compared to the
total background radiation in the Netherlands, which is ~2.5 mSv/year. For
comparison, the radiation dose during a flight higher than 10 km is 0.005
mSV*h-1.
The benefit of participation in this study is that all food will be provided
for 14 days.